1. Field of the Invention
The invention relates to a transmitter device with two insulating couplings, and designed to be mounted on a tubular string in a borehole such as an oil well during testing or production, for the purpose of transmitting information to a receiver on the surface.
2. Background of Invention
While an oil well is being drilled, use is frequently made of a wireless technique of transmitting information, which technique consists in using the fluid that then fills the well to transmit hydraulic pressure waves conveying said information. However, that technique of transmitting information is not applicable to a well while it is being tested or is in production.
Document U.S. Pat. No. 2,400,170 shows that proposals have been made for many years to convey information from down a borehole up to the surface by using an electromagnetic wave which is modulated as a function of the information to be conveyed. To this end, an electrical signal is applied between two lengths of a tubular metal string placed in the borehole on opposite sides of an insulating coupling mounted in the string.
Document FR-A-2 618 912 describes an insulating coupling suitable for a practical application to a well that is being drilled of the technique of conveying information in the form of electromagnetic waves. The insulating coupling includes a ceramic coating that is interposed between complementary cylindrical threads formed on two lengths of the tubular string. Further insulation is obtained by an internal insulating coating and by external insulating coating providing continuous electrical insulation with the ceramic coating. These two insulating coatings overlie the inside and outside surfaces of the tubular string over a certain length from the insulating coupling.
Nevertheless, the insulating coupling described in document FR-A-2 618 912 which was designed for use in a well while the well is being drilled, is not adapted to transmitting information in a well that is under test or that is in production.
Thus, the annular string or tubing in a well that is being tested or that is in production must have a central bore that is completely free, in order to enable a specific apparatus for the purpose of to be subsequently inserted therethrough. With an insulating coupling such as that described in document FR-A-2 618 912, this requirement is incompatible with the dimensions that need to be given in practice to the conductors that are used for applying a modulated alternating electrical signal to opposite sides of the coupling.
Assuming, for example, that the power source used for emitting the electrical signal is placed below the sealed coupling, it becomes practically impossible for the sealed coupling to have electrical conductors connected to the length of string situated above the coupling by passing through the coupling without unacceptably reducing the mechanical strength of the production string at this location, given the large section required of the conductors because of the power they need to convey above the coupling in order to ensure that a usable signal is received at the surface.
In addition, the sealed coupling described in document FR-A-2 618 912 has a structure that makes it possible to provide good electrical insulation between the portions of the drilling string situated above and below the coupling, but which is not suitable for withstanding the high pressure difference (up to 10.sup.8 Pa) that may exist between the inside and the outside of the annular string in a well being tested or in production.
A particular object of the invention is to provide a novel type of transmitter device suitable for being mounted in a test or production string in an oil well, enabling a relatively high power modulated alternating electrical signal to be applied between two lengths of the string that are electrically insulated from each other.
Another object of the invention is to provide a transmitter apparatus enabling the central bore of the string to be left completely free.
A further object of the invention is to provide a device capable of withstanding the large pressure differences that may exist between the inside and the outside of the string without causing the string to lose its sealing properties.